Foot-and-mouth disease (FMD) is a highly contagious, debilitating disease that afflicts cloven-hoofed animals. The causative agent, foot-and-mouth disease virus (FMDV), is a small animal virus of the picornavirus family having a single stranded positive sense RNA genome of about 8000 nucleotides.
In the past, vaccines have been produced for FMD using either an inactivated virus or a live attenuated virus. These approaches, although generally effective, have not been problem-free. Occasions in which the virus was incompletely inactivated or insufficiently attenuated can and have given rise to FMD outbreaks. A synthetic FMDV vaccine which would eliminate all aspects of virus handling in production and vaccination is perceived as a most desirable alternative [D. Rowlands, Endeavor 8 (3), 123-127 (1984)].
Studies of FMDV over the last several years have established that the picornavirus contains primarily four capsid proteins designated VP.sub.1, VP.sub.2, VP.sub.3, and VP.sub.4. The actual labelling of these VP proteins has been subject to a measure of inconsistency. The relevant protein insofar as this invention is concerned has been designated VP.sub.3 by some research groups [see, e.g., Kleid et al., Science 214, 1125 (1981)] while being labelled VP.sub.1 by others [see, e.g., Clarke et al, FEBS Letters 157, 261 (1983)]. For purposes herein, this protein is designated VP.sub.1.
VP.sub.1 has been the subject of considerable investigation and structural analysis, it having been found to elicit a neutralizing antibody response in swine [J. Laporte, J. Grosclaude, J. Wantyghem, S. Bernard, P. Rouze, C. R. Acad. Sci. 276, 3399 (1973)] and to protect both swine and cattle from FMDV infection [H. L. Bachrach, D. M. Moore, P. D. McKercher, J. Polatnick, J. Immunol. 115, 1636 (1975)]. A biosynthetic fusion polypeptide of TrpLE and VP.sub.1 upon multiple vaccination has been reported to protect cattle and swine against a viral contact challenge [D. G. Kleid, D. Yansura, B. Small, D. Dowbenko, D. Moore, M. Grubman, P. McKercher D. O. Morgan, E. V. Robertson, H. L. Bachrach, Science 214, 1125-1129 (1981).
Strohmaier et al., J. Gen. Virol. 59, 295-306 (1982), by chemical and enzymatic digestion of VP.sub.1, have concluded that major immunogenic regions capable of raising neutralizing antibodies are present at amino acid sequences 146-154 and 201-213.
Bittle et al., Nature 298, 30-33 (1982) chemically synthesized peptides corresponding to regions 141-160 and 200-213 of the VP.sub.1 polypeptide of FMDV and demonstrated the ability of these peptides to produce high levels of neutralizing antibody in guinea pigs and rabbits. A report on this work is also contained in the PCT International patent application No. WO 83 03,547, published Oct. 27, 1983.
A major theme which runs throughout all of the foregoing work with the small peptide sequences is the belief and finding that their immunogenic effect, if present at all, is achieved only upon the linking of the peptide to a high molecular weight carrier, most often keyhole limpet hemocyanin (KLH) carrier.
The aforementioned PCT application No. WO 83 03,547 appears to suggest the converse. In fact, the data presented in the publication establish that a small peptide, in monomeric and uncyclized form, is incapable of eliciting a neutralizing index sufficient to achieve viral protection. Specifically, the publication states that a neutralizing index of about 1.5 or greater is required to protect an animal against the virus and that the neutralizing index of the monomeric, unconjugated peptide having a sequence which corresponds substantially to the amino acid residue sequence in positions about 141 to about 160 of OlK FMDV is approximately 0.5. This result is to be compared with neutralizing indices of about 3.7 to about 3.9 reported for the same peptide when conjugated to KLH.
A class of novel, monomeric, unconjugated, small peptides has now been discovered. These peptides, administered free of any carrier, are capable of eliciting an unforeseen markedly enhanced neutralization response against FMDV. These peptides have induced unprecedented protection in cattle with single vaccination and complete protection after repeat immunization. Until the present discovery, this latter achievement has been proven only with antigens approximately five times larger in size than these newly discovered peptides.